Breathable backsheet design for disposable absorbent articles

ABSTRACT

The present invention relates to disposable absorbent articles such as baby diapers or sanitary napkins having a breathable yet liquid leakage retarding backsheet which comprises an inner and an outer layer. The inner layer being closer to the absorbent structure of the article is a formed film with directional liquid transport characteristics while the outer layer is a fibrous fabric.

FIELD OF THE INVENTION

The present invention relates to disposable absorbent articles such asbaby diapers or sanitary napkins having a breathable yet liquid leakageretarding backsheet which comprises an inner and an outer layer. Theinner layer being closer to the absorbent structure of the article is aformed film with directional liquid transport characteristics while theouter layer is a fibrous fabric.

BACKGROUND OF THE INVENTION

Disposable absorbent articles such as baby diapers, adult incontinenceproducts, sanitary napkins and panty liners are well known in the art.These articles have a wearer facing side through which they typicallyabsorb liquids discharged by the wearer. The liquid is stored in anabsorbent structure. Liquid leakage from the article through the surfaceopposite the wearer facing side is usually prevented by incorporating aliquid impermeable backsheet on that side.

It is also well established in the art that a backsheet allowing gaseousfluid (air) communication with the environment, usually referred to asbreathability, is highly desirable. Breathability improves with theamount of air permeating through a backsheet. This amount isproportional to the open area (the sum of the area of all apertures) inthe backsheet. Obviously too many and particularly too large aperturesin the backsheet lead to compromising the liquid leakage prevention,which is the primary function of a backsheet.

Many suggestions how to provide breathable backsheets have been recordedin the art. Numeral attempts of combining the mutual contradictingfeatures of gas permeability and liquid impermeability have beendocumented in patents and patent applications. However the lack ofcommercially available breathable disposable absorbent articlesindicates that the technology so far suggested has not provided an allaround satisfactory result for the desired technical requirements atcommercially acceptable condition. More often than not satisfaction ofone desired feature went to such an extreme that the respective otherfeature was not properly satisfied any longer.

For example sanitary napkins with very high breathability at the cost offrequent liquid leakage (leading to soiling of the undergarments of awearer) cannot be considered satisfactory. On the other hand satisfyingthe liquid leakage problem properly usually resulted in almostimpermeable, that is non-breathable, backsheets. In particularmicroporous films which have no macroscopic apertures are not liquidpermeable. But microporous films only allow air communication bydiffusion which is an order of magnitude less than the achievedbreathability with the current invention.

Good progress has been made in the field of formed films havingdirectional liquid transport wherein liquid transport over a certainpressure drop across a formed film is better in one direction versus theother. These so called one way formed film materials have found wideusage as liquid permeable topsheets after being treated with surfactantin the field of sanitary napkins and panty liners, but also for diapersand incontinence products. Alternative topsheets of fibrous fabricpolymers typically have no directional liquid transport. They are alsotreated with surfactant but have an intrinsic hydrophobic behaviour whenbeing used as topsheets for absorbent articles after the surfactant hasworn off.

Combinations of breathable and liquid permeable sheets in order toprovide a certain liquid impermeability while satisfying the desire forbreathable backsheets have already been suggested for example in U.S.Pat. No. 3,881,489. In this disclosure a breathable backsheet isprovided by confining an outer layer of formed film material havingsurface aberrations with apertures therein and an inner layer of a papertissue having a high void volume and having been made hydrophobic byimpregnating it with a paraffin wax. This document does not disclose thedesire for using a directional liquid transport type polymeric filmstructure with a hydrophobic fibrous fabric layer made of polymericmaterial.

Other prior art attempts to provide breathable backsheet assembliescomprising more than one layer are e.g. documented in U.S. Pat. No.4,341,216, EP-A-109 126 or EP-A-203 821. Neither of these disclosuresprovides constructions of breathable backsheets similar to the presentinvention.

Single layer breathable backsheets are known for example fromGB-A-2184391, GB-A-2184390, GB-A-2184389, U.S. Pat. Nos. 4,591,523,4,839,216 or EP 156471.

In PCT publication WO 9309744 absorbent articles which have a hybridtopsheet are disclosed. The hybrid topsheet comprises a non-woven fabricoverlaid by a formed film were the non-woven fabric is folded around theedges but not fully covering the second surface of the formed film. Thisstructure is used as the topsheet to facilitate a liquid absorption incontrast to breathability. However it would of course providebreathability if it was left in air communication with the environment.This disclosure teaches to use the structure as a topsheet which istypically rendered hydrophilic. Further the disclosed topsheet structurehas a formed film outer layer and a non-woven inner layer in respect tothe absorbent structure. Therefore this publication also does notdisclose the structures according to the present invention.

In unpublished, pending applications U.S. Ser. Nos. 08/042,364;08/042,365 and 08/042,345 all filed on Feb. 4, 1993 other topsheetscomprising a formed film and non woven fabric are disclosed. Howeverthese constructions are rendered hydrophilic for the purpose of usingthem as topsheets receiving liquid to be transported towards theabsorbent core rather than to use them on breathable backsheets.

It has now been found that combining a gas permeable, hydrophobic,polymeric fibrous fabric and an apertured formed film having adirectional liquid transport phenomena provides a particularly desirablebreathable backsheet. Backsheets according to this construction haveexceptionally good breathability due to the large open area and thecombination of formed film and fibrous fabric allows to select thelayers so as to provide the desired liquid leakage retarding function.Therefore the present invention provides an non-leaking breathablebacksheet and absorbent articles comprising this backsheet.

It is therefore an objective of the present invention to provideabsorbent articles in particular sanitary napkins or panty liners havinga superior breathability as defined by free gas permeability of thebacksheet while simultaneously retarding liquid leakage through thatbacksheet to such an extend that the user of such products does notexperience a recognisable difference between a liquid impermeablebacksheet and the breathable backsheet according to the presentinvention.

An additional objective satisfied by the present invention is also toprovide a backsheet which has a desirable and user preferred fibrousoutside surface.

CROSS REFERENCE

Another patent application is being filed on the same date as thisapplication. It is entitled “Breathable dual layer backsheet design fordisposable absorbent articles” by M. Depner and M. Divo and alsoassigned to “The Procter and Gamble Company”.

DESCRIPTION OF THE INVENTION

The present invention relates to breathable absorbent articles such asbaby diapers, adult incontinence products, sanitary napkins or pantyliners. Typically such products have a topsheet, a backsheet and anabsorbent core between the topsheet and the backsheet. The articlesaccording to the present invention have a breathable backsheetcomprising an inner layer and an outer layer where the inner layer iscloser to the absorbent core than the outer layer. The outer layercomprises a hydrophobic, gas permeable fibrous fabric layer composed ofpolymeric fibres such as polymeric non-wovens well known in the art ofabsorbent articles.

The inner layer comprises a hydrophobic gas-permeable aperturedpolymeric film having a directional liquid transport phenomena. The filmhas a first and a second liquid transport direction which are oppositeto each other. The first liquid transport direction is from the outerlayer towards the absorbent core. Liquid transport in the firstdirection is larger than liquid transport in the second direction whenmeasured under an identical pressure drop across the apertured film.

It is preferred that the breathable backsheet of the absorbent articleprovides a threshold pressure below which a test liquid does notpermeate in the second liquid transport direction through the backsheet,thereby providing a “zero leakage” threshold. This threshold will bedependent on the usage circumstances of the product, e.g. babies willnot provide a pressure onto their diaper when sitting as large as womenonto their panty liners when bicycling.

For sanitary napkins or panty liners the respective pressure thresholdhas been found to be equivalent to 45 g/cm² (i.e. 4414.5 Pa.) accordingto the test protocol disclosed in the example below. The test liquidused to identify test leakage comprises a saline solution consisting of2 g urea, 0.9 g sodium chloride, 0.06 g calcium chloride and 0.11 ghydrated magnesium sulphate in 100 ml distilled water. This solution isadjusted for surface tension by adding surfactant to better simulatebodily discharges other than urine. The reduced surface tension of thesolution 29+/−1 mN/m while the saline solution without surfactant has asurface tension of more than 60 mN/m.

The fibrous fabric layer of the outer layer preferably has a basisweight of 10 to 100 g/m² preferably 15 to 30 g/m². The fibres can bemade of any hydrophobic polymeric material usual in the art of makingfibrous fabric layers. Depending on the circumstances of the ultimateuse and manufacturing of the breathable absorbent article fibres ofpolyethylene, polypropylene, polyester, polyacetat or combinationsthereof (intra- and inter-fibres combinations) have been found useful.The fibres are preferably spunbonded, carded or melt blown. The fabriclayer most preferably comprises a matrix of spunbonded fibres covered onone or both sides with meltblown fibres but can also be provided by anyother typical technology used in the art.

The apertured film according to the present invention can be any ofthose well known in the art. This includes in particular, but is notlimited to those films disclosed in U.S. Pat Nos. 3,929,135, 4,151,240,4,319,868, 4,324,426, 4,342,314 and 4,591,523.

The apertured film comprised in the inner layer of the breathablebacksheet preferably has funnel shaped apertures similar to thosedescribed e.g. in U.S. Pat. No. 3,929,135. The apertures maybe circularor non-circular but have a cross sectional dimension at one end of thefunnel which is wider than the opening at the other end of the funnel.The direction from the larger funnel opening towards the smaller openingis of course parallel to the first liquid transport direction. The openarea of the apertured film is typically more than 5%, preferably in therange of 10% to 35% of the total film surface. The apertured films canbe made of any material typical in the art but preferably is made of apolymer similar to those used for the fibrous fabric layer.

The minimum hydraulic diameter of the apertures in the film should be assmall as possible while still providing sufficient gas permeabilitywithout hydraulic blockage of the apertures. A hydraulic diameter of aslittle as 0.2 mm, preferably 0.3 to 0.7 mm has been found possible inthe context of the present invention. Hydraulic diameter for noncircular apertures is the diameter that a circular aperture with thesame cross section would have. Diameter is always determined in theplane of smallest cross section of an aperture.

In particularly preferred embodiments of the present invention thelayers of the breathable backsheet are joined to each other across lessthan the total area which is coextensive with the absorbent core.Particularly inner and outer layers which are unattached across half ofthe area, most preferably across the total area which is coextensivewith absorbent core have been found beneficial to leakage preventionwithout breathability reduction. Of course the outer layer and innerlayer of the backsheet need to be combined somewhere to create thebreathable backsheet combination of the present invention. Thiscombining can for example be provided in the periphery outside the areacoextensive with the absorbent core or in a pattern of lines or dotsacross the whole area coextensive with the absorbent core which patterndoes not cover any significant area itself.

The present invention as indicated above can be used beneficially in thecontext of many different absorbent articles. However sanitary napkinsand especially thin panty liners are particularly susceptible to thepresent invention. Sanitary napkins or panty liners having a thicknessof 3 mm or less and preferably 2 mm or less benefit especially well fromthe breathable backsheet of the present invention.

The disposable absorbent articles according to the present invention canhave all those other features and parts which are typical for productsin the context of their intended use. They comprise typically theabsorbent structure which can be a fluffy fibrous absorbent corecomprising also hydrogel particles if desired, laminated tissues with orwithout particulate materials including hydrogel particles or odourcontrol particles. The absorbent core fibres can be any of those knownin the art including cellulose fibres or polymeric fibres renderedabsorbent or even non-absorbent matrix fibres. Also tissues ofsufficient basis weight and absorbency can be used as the absorbentstructure according to the present invention. E.g. the tissue used inthe method described below to measure wet-through can be used in thecontext panty liners.

Also a topsheet or wearer contacting layer through which the liquids tobe absorbed penetrate to the absorbent structure is typicallyincorporated in articles according to the present invention. Thetopsheet or wearer contacting layer can be provided by any of thosematerials and techniques known in the art including formed films andnon-woven fibrous fabrics similar to those described herein above.

EXAMPLES

For testing the following examples the wet-through method as describedbelow was used. It follows conceptually the wet-through method disclosedin U.S. Pat. No. 3,881,489 but relates to the backsheet wet-throughprevention ability.

Test Preparation

The breathable backsheet to be analysed is provided with a generallyhydrophilic, test liquid absorbing tissue on the inside surface. Thetissue is placed without exerting pressure on the backsheet. Thensamples of 5 cm times 5 cm are cut and compressed under a load of 40g/cm² (or 1.0 kg per sample). Not required for the test but desirablefor easy operation of the test is an additional layer of non wovenfabric, or formed film on top of the absorbent tissue.

For repeatability the samples are kept for at least 4 hours at testconditions. Test conditions are 23° C. and 50% relative humidity. Thetest liquid is prepared by mixing 100 ml of distilled water, 2 g urea,0.9 g NaCl, 0.11 g MgSO₄×7 H₂O and 0.06 g CaCl₂. The test liquid then isadjusted to a surface tension of 29+/−1 mN/m to resemble particularlythe condition exhibited by women with vaginal discharge. To resembleother discharges the surface tension can be kept unaltered at about 60mN/m.

Test Materials

Typical test materials are e.g. a tissue designated 609912, of 99 g/m²obtainable from the Merfin Hygienic Products company, in Delta, B.C.,Canada. Generally the tissue should be not thicker than 1 mm.

The blotter paper can be any good absorbent blotter paper commerciallyavailable. For the tests below blotter paper designated Absorbent biancoNo 30 (220 g/m²) of Cartiera, Favini in 36028 Rossano Veneto (VC), Italywas used. The blotter paper can be cut into larger areas than the 5 cm×5cm sample size.

The desirable non-woven or formed film shall not be absorbent and shallhave no surface residue wash-off which would alter the test liquidcomposition. This can be achieved by washing the material with testliquid and drying it prior to it's use.

Test Liquid Quantity

The test liquid quantity to be used in the test is determined as theamount necessary to saturate the absorbent tissue. This can bedetermined by preparing tissue samples of 5 cm×5 cm, preferably largerbut with a known surface area, of the tissue to be used in thewet-through test. The weight of the tissue sample is determined.

The tissue is then placed on a nylon net having rectangular and diagonalrunning threads stretched inside a frame. The net forms rectangles of 5cm×4.5 cm which are split into 4 equal parts by threads connecting thecorners diagonally. The frame is preferably of a water repellentmaterial such as a hydrophobic polymer.

The net together with the tissue samples are then immersed in destilledwater of 23° C. (+/−1° C.) for 30 seconds (+/−3 seconds). Then the nettogether with the tissue is left to drain/drip in a horizontal positiononly under gravitational forces for 120 seconds (+/−5 seconds). Then theweight of the tissue together with the absorbed water is measured. Thescales for the weight measurements should be accurate to 0.1 gram.

The amount of absorbed water is calculated (wet weight minus dry weight)and divided by the surface area of the tissue sample. This value ismultiplied by the surface area of 25 cm² of the wet-through test samples(5 cm×5 cm) to result in the test liquid quantity to be used.Statistical analysis should be used to ensure an accuracy of +/−10% ofthe test liquid quantity value within a +/−3 Sigma range of the testseries (adjusted for surface area of 25 cm².

Test Method to Determine Wet-through at Defined Load

The samples are placed on an absorbent blotter paper with the breathablebacksheet adjacent the blotter paper. The size of the blotter papershould be larger than the test sample size. The blotter paper isconditioned as the test samples are and its weight prior to the test istaken.

Then the defined test liquid quantity is placed in the centre of thesample. The flow rate when placing the liquid should be about 1 ml/10sec., i.e. the test liquid should be placed onto the sample in about 20sec. for 2 ml loadings. After 2 minutes a load of 40 g/cm² is applied tothe wet sample for 15 seconds. Typically the minimum load is 20 g/cm² or0.5 kg per sample, however for simulating a stress condition a load of40 g/cm² is applied. After removal of the load the blotter paper ischecked for wet-through.

If there is wet-through the occurrence is noted as qualitative result orthe weight of the blotter paper with the leaked absorbent liquid whichhas been absorbed is measured. The difference of the second measurementto the original dry weight of the blotter paper is the amount ofwet-through which provides a quantitative result.

Care must be taken to not introduce side leakage e.g. due to an unevenliquid migration from the centre of the sample. The samples must bestored horizontally during the test. If side leakage occurs anyway atthe edges of the sample and wet trough is observed the quantitativeamount of wet-through outside the sample area is to be disregarded. Ifall test samples have side edge leakage without wet-through occurring adifferent tissue should be used to confirm the test result. If theresult is consistent then the qualitative and quantitative results areno wet-through.

Results

At least 10 samples should be measured and averaged. Statisticalanalysis should be used to confirm that the average result isstatistically correct within a 95% confidence interval.

Test Method to Determine Wet-through Threshold Load

This method is identical to the test method at constant load except thatthe load starting from 20 g/cm2 is gradually increased by increments of5 g/cm² (125 g per sample). The measurement of wet-through quantity isnot required but the test person needs to determine the lowest load atwhich wet-through occurs. The threshold load is defined as the lowestload at which wet-through occurs minus 5 g/cm². If wet-through occurs at20 g/cm² already the breathable backsheet is reported to be leakingwithout threshold load.

Test Products

Backsheets according to the present invention were constructed from thefollowing raw material:

non-woven fabric of 28 g/m² having a spunbonded layer of 14 g/m² and amelt blown layer of 14 g/m² obtainable from Corovin GmbH, Peine, Germanyunder the designation MD 2000.

polyethylene formed film according to U.S. Pat. No. 3,929,135 obtainablefrom Tredegar Film Products B.V., Kerkrade, The Netherlands. The filmhas circular funnel shaped apertures with an open area of 19%, anembossed thickness of 0.48 mm (funnel height) and an aperture diameterof 0.465 mm.

The backsheet according to the invention is prepared by joining the filmwith the funnels pointing towards the absorbent structure (for thistesting the absorbent structure was the 99 g/m² Merfin tissue describedsupra) and the fabric with the melt blown layer being oriented to becomethe inter face between formed film and fabric.

In an example according to the present invention the fabric and the filmare overlaying each other without attaching the two layers. The testliquid was used at 2 ml per sample.

Threshold Pressure Test with Reduced Surface Tension Test Liquid

First leakage observed at 50 g/cm² Threshold pressure 45 g/cm²

Wet/through Test

Wet/through qualitative test were conducted at 40 g/cm² and 220 g/cm²with test liquid (surface tension 29 mN/m) and test liquid withoutsurfactant (surface tension 62 mN/m). The results provide a valuabledisplay of the synergistic effect of the breathable structure accordingto the present invention. In the table below “Y” indicates wet-throughoccurrence (quantitative result in % of test liquid in brackets) while“N” indicates no wet-through (0%).

Load 40 g/cm² 220 g/cm² surface tension 62 mN/m (test liquid withoutsurfactant) example according to the N N present invention film withfunnels N Y  (14.5%) fabric with melt blown layer N N surface tension 29mN/m (test liquid with surfactant) example according to the N Y (<50%)present invention film with funnels N Y (>50%) fabric with melt blownlayer Y (35%) Y (>50%)

What is claimed is:
 1. Breathable absorbent article comprising atopsheet, a breathable backsheet and an absorbent core between saidtopsheet and said backsheet, said backsheet comprising an inner layerand an outer layer, said inner layer being closer to said absorbent corethan said outer layer, said article being characterised in that saidouter layer comprises a hydrophobic, gas-permeable fibrous fabric layercomposed of polymeric fibres and said inner layer comprises ahydrophobic, gas-permeable apertured polymeric film, said apertured filmhaving a first liquid transport direction and a second liquid transportdirection opposite said first liquid transport direction, said innerlayer being oriented such that said first direction is from said outerlayer towards said absorbent core, said apertured film allowing a liquidtransport in said first liquid transport direction which is larger thanthe liquid transport in said second liquid transport direction under anidentical pressure drop across said apertured film.
 2. Breathableabsorbent article according to claim 1 wherein said article is asanitary napkin or panty liner having a thickness of 3 mm or less. 3.Breathable absorbent article according to claim 2 wherein saidbreathable backsheet has no liquid transport in said second directionunder a pressure of a load of 45 g/cm² or less for an aqueous salinesolution consisting of 100 ml distilled water, 2 g urea, 0.9. g NaCl,0.11 g MgSo₄×7H₂O, 0.06 g CaCl₂, the saline solution is adjusted withsurfactant to 29+/−1 mN/m.
 4. Breathable absorbent article according toclaim 2 wherein said article is a sanitary napkin or panty liner havinga thickness of 2 mm or less.
 5. Breathable absorbent article accordingto claim 1 wherein said fibrous fabric layer is made of fibres ofpolyethylene, polypropylene, polyester, polyacetate or combinationsthereof and preferably said fabric layer comprises a matrix ofspunbonded fibres covered on one or both sides with meltblown fibres. 6.Breathable absorbent article according to claim 1 wherein said aperturedfilm comprises funnel shaped apertures wherein the direction from thelarger funnel opening towards the smaller opening is parallel to saidfirst liquid transport direction.
 7. Breathable absorbent articleaccording to claim 1 wherein said apertured film has an open area ofmore than 5%, of the film's total surface.
 8. Breathable absorbentarticle according to claim 7 wherein said apertured film has an openarea in the range of 10% to 35% of the total film surface.
 9. Breathableabsorbent article according to claim 1 wherein said inner layer and saidouter layer, across at least 50% of the area which is coextensive withsaid absorbent core, are not attached to each other.
 10. Breathableabsorbent article according to claim 9 wherein said inner layer and saidouter layer, across at least 90% of the area which is coextensive withsaid absorbent core, are not attached to each other.
 11. Breathableabsorbent article according to claim 1 wherein said fibrous fabric layerhas a basis weight in the range of 10 to 100 g/m².
 12. Breathableabsorbent article according to claim 11 wherein said fibrous fabriclayer has a basis weight in the range of 15 to 30 g/m².
 13. Breathableabsorbent article according to claim 1 wherein said apertured film hasapertures with a minimum hydraulic diameter of 0.2 mm.
 14. Breathableabsorbent article according to claim 13 wherein said apertured film hasapertures with a minimum hydraulic diameter in the range of 0.3 mm to0.7 mm.
 15. Breathable absorbent article according to claim 1 whereinsaid inner layer and said outer layer, across the whole area which iscoextensive with said absorbent core, are not attached to each other.